The present invention relates to a precisely flow-controlling pump used in an analyzing instrument for pumping a liquid specimen with a stable flow rate and, more particularly, to a precisely flow-controlling pump having control flow means used as a pump for a liquid chromatographic apparatus.
In most of pumps used in liquid chromatographic apparatus, the volume inside cylinder is changed by reciprocating a plunger forth and back in the cylinder. Liquid can be always delivered by connecting two pumps in series and independently moving the plungers of the two pumps. The pumps have a structure that liquid is discharged from a delivery port in such a manner that a checking valve attached in a suction port in the cylinder opens when the plunger moves toward a direction to increase the volume inside the cylinder, and the checking valve attached in the suction port in the cylinder closes and a checking valve attached in the delivery port opens when the plunger moves toward a direction to decrease the volume inside the cylinder.
In such pumps, the rotating speed control to drive the cylinders and the geometrical accuracy in the cams converting the rotating movement of the motor into the reciprocal movement of the plungers strongly effect the constancy of delivery flow rate of liquid. As for instable factors, there is malfunction mainly caused by bubbles entering in the liquid or operation lag in the checking valves.
In order to decrease the malfunction, the prior art of compensating pulsating flow by detecting change in delivery liquid pressure is proposed in Japanese Patent Application Laid-Open No. 55-128678 (1980), Japanese Patent Application Laid-Open No. 63-105285 (1988), Japanese Patent Application Laid-Open No. 60-11690 (1985) and Japanese Patent Application Laid-Open No. 58-105028 (1983).